The present invention relates to a method of preparing gold(I) mercaptides of the formula AU--S--R by reacting a gold(I) halogenide sulfide complex, obtainable from a gold(III) halogenide or gold(III) halogen complex and a sulfide, with a mercapto compound of the formula R--S--H.
Aliphatic, cycloaliphatic and aromatic gold(I) mercaptides of the general formula Au--S--R, in which R represents an organic group and in which the sulfur is bound to an aliphatic, cycloaliphatic or aromatic C-atom of the R group have long been known. The scope and definition of R, the organic group, are well recognized in the art. Such mercaptides are used in so-called gold preparations for the gilding of solid articles, especially ceramic materials, as components in luster preparations, and for producing strip conductors in integrated circuits.
A survey of the methods of preparing gold(I) mercaptides and of their use is contained in German patents 12 84 808, 12 86 866, 12 98 828; and U.S. Pat. Nos. 4,221,826 and 3,245,809 and 3,163,665 and 2,984,575 which U.S. patents are incorporated by reference, especially for the definition of R, the organic group.
According to these patents, two methods are known for preparing gold(I) mercaptides of the formula R--S--Au:
(A) 3 RSH+AuX.sub.3 .fwdarw.RSAU+R--S--S--R+3 HX PA0 (B) 2 R'.sub.2 S+AUX.sub.3 +H.sub.2 O.fwdarw.AuX.multidot.R'.sub.2 S+R'.sub.2 SO+2 HX
AUX.multidot.R'.sub.2 S+RSH.fwdarw.RSAU+R'.sub.2 S+HX.
Method A comprises the reaction of a gold(III) halogenide with three equivalents of mercaptan in the presence of an organic solvent. The gold(III) halogenide is reduced by means of two equivalents of mercaptan with the formation of disulfide to produce the gold(I) halogenide, which reacts immediately in the presence of the third equivalent of mercaptan to form the gold(I) mercaptide.
Method A is disqualified by the loss of two equivalents of mercaptan. As a rule, the mercaptans are chemically tailor-made compounds designed for their intended application and are thus too expensive to be utilized as a reducing agent. In addition, the separation of the gold(I) mercaptide from the disulfide is very expensive, requires large amounts of organic solvent, and frequently results in yield losses.
In the case of method B, the reduction of the gold(III) halogenide and its complex formation is carried out in the presence of at least one equivalent of water and two equivalents of a dialkyl sulfide. The gold(I) halogenide dialkyl sulfide complex compound formed thereby reacts with the mercaptan to form the gold(I) mercaptide with liberation of the dialkyl sulfide. One equivalent of dialkyl sulfide accumulates as a byproduct of the reduction stage.
In method B, a dialkyl sulfide such as dimethyl-, diethyl-, dibutyl- or dioctyl sulfide (which is more economical than the tailor-made mercaptan of method A) can be used as the reducing agent. In method B, the reaction mixture normally contains, after the reaction per equivalent gold(I) mercaptide, one to two equivalents of dialkyl sulfide and one equivalent of dialkyl sulfoxide which must be separated from the gold(I) mercaptide.
A disadvantage of method B is the fact that the previously used dialkyl sulfides are found to be undesirable from a work environment viewpoint on account of their penetrating odor. Even in the gold(I) mercaptides produced, traces of still-remaining dialkyl sulfide cause considerable odor problems.
Whereas the sulfoxides of the previously named dialkyl sulfides are sufficiently water-soluble and can generally be satisfactorily separated during the workup of the reaction mixtures from the gold(I) mercaptides, the alkyl sulfides themselves are non-water-soluble. The alkyl sulfides must therefore be separated by means of organic solvents from the gold(I) mercaptide. These solvents must be carefully selected as they should dissolve only the dialkyl sulfide but not the gold(I) mercaptide. The dialkyl sulfides can usually not be completely separated by washing out the usually solid gold(I) mercaptides, as the examples in the above-named patents show. A reprecipitation is required in most instances. However, such a reprecipitation is limited to gold(I) mercaptides soluble in organic solvents.
In an attempt to improve the previously known methods, Applicants also used water-soluble dialkyl sulfides (such as bis-hydroxyethyl sulfide and 2,2'-thiodiacetic acid) in the methods in question. It turned out as a result that the desired reactions did take place but the solubility of the named sulfides in organic solvents (such as dichloromethane, toluene, acetone, and ethanol) is so great that the above-mentioned problems occur in the preparation, isolation and purification of readily soluble gold(I) mercaptides in the presence of organic solvents.